The present invention relates to a color image forming apparatus and, more particularly, to a color image forming apparatus of the type including a plurality of photoconductive elements.
In a color electronic copier or like image forming apparatus which uses a plurality of photoconductive elements for forming a color image, image components which should be in register are often deviated from each other in an intended direction of paper feed due to the influence of various factors such as the positions and peripheral speeds of the photoconductive element, the positions where the photoconductive elements are exposed imagewise, and the linear velocity of a transfer belt. It has been customary to guarantee such various factors by the accurate machining and assembling of individual parts. This kind of approach, however, not only requires extra costs but also needs readjustment due to the variation of the individual factors and scattering which is ascribable to the replacement of parts.
An alternative approach heretofore is locating a plurality of sensors in front of each transfer station so that the sensors may sense a paper to provide a writing timing which is associated with a particular color, as disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 59-155870. Even with this approach, it is difficult to control the positional deviation of color image components to below a predetermined upper limit (about 0.15 millimeter).
The positional deviation may be corrected by sensing particular patterns of different colors which are previously marked on a transfer belt. With this scheme, however, there is a fear that the patterns on the belt fail to be accurately read due to smears on the belt and/or those on a sensor, scattering of sensitivity among sensors, etc. Moreover, when the belt has a joint or scratches, even the joint or the scratches are sensed to obstruct accurate measurement of the pitch of the patterns. The accurate measurement is also unattainable when the line width of the patterns is changed as a result of changes in process conditions.
Further, the timing for starting writing an image component may be corrected on the basis of a positional deviation sensed as discussed above. This kind of scheme has a shortcoming that when the positional deviation is increased beyond a predetermined correctable range, inhibiting all the image forming modes available renders the machine practically unusable. Even when the deviation is greater than a predetermined value, image forming modes other than the mode which uses the color concerned are usable. In addition, even if the deviation associated with any color is greater than the predetermined value, a single color mode associated with that color is usable.